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Planetary News: SETI (2004)

Can a Star's Glow Reveal an Advanced Civilization?

The great sphere had been in place since time immemorial. There was a time, ancient legends told, when one could see stars in a dark night sky, a time when The People had lived on a single planet orbiting in the open vastness of space. The great sphere had done away with all that, enclosing all the planets of the inner planetary system and the Sun itself in a giant solid shell. It had been the greatest project ever undertaken by The People, and it had been well worth the effort. Enclosed by the great sphere, the Sun was now a source of unlimited energy, forever solving the most troublesome problem of this advanced technological civilization. These days one could see a red glow in the night sky, a sign that the enormous shell was absorbing the heat emanating from the Sun and trapped within the great sphere. The glow had been there for so long that no one could now imagine a world not enclosed by the great sphere and its dull red aura.

Are there such enormous artificial spheres, surrounding worlds inhabited by intelligent beings? Obviously, we do not know. But in a famous article published in Science in June of 1960 physicist Freeman Dyson suggested that advanced civilizations might in fact build such structures.

Given that humanity has only just stepped into its own technological age, Dyson argued, it is highly likely that an alien civilization would be more advanced than humans by several orders of magnitude. Based on human experience, a good measure of a civilization’s technological level is the amount of energy it consumes. Since the ultimate source of energy for a planetary civilization is its sun, it is reasonable to expect that the most advanced civilizations would fully exploit this almost limitless resource. This can be done, according to Dyson, by fully enclosing the sun and the planets in a giant artificial sphere, which would contain the sun’s radiation and make it available for exploitation. He calculated that using all the matter of a Jupiter-sized planet, it would be possible to create such a sphere at twice the distance of the Earth from the Sun, its walls several meters thick at every point.

The crucial fact about such “Dyson Spheres” is that they could actually be detectable from Earth. According to the laws of thermodynamics, the huge amount of energy absorbed by the sphere would have to be dissipated as heat. This means that to an observer from Earth, Dyson Spheres would appear to glow at the infrared part of the spectrum. While full Dyson Spheres, which completely enclose their stars, may not be detectable from the Earth, partial or incomplete Dyson Spheres should be visible. To identify them, Dyson suggested, SETI researchers should look for normal stars that produce an excess of infrared radiation.

How to Search for Dyson Spheres

Dyson made his proposal four and a half decades ago, at the dawn of the space age. Several generations of SETI projects have come and gone since then, but Dyson’s provocative idea has never lost its power to surprise and fascinate. Nevertheless, few attempts had been made over the years to test Dyson’s radical idea.

Part of the problem is simply one of confirmation: suppose SETI scientists detected a star that emits an excess of infrared radiation. What then? The presence of such radiation in itself hardly proves the presence of a Dyson Sphere. There could be a natural explanation for this radiation that does not involve intelligent beings, and which would seem more plausible to many scientists. Because of this, any identification of a potential Dyson Sphere would have to be followed up by a traditional SETI observation, looking for radio or light transmissions from the star. Only a direct confirmation of an intelligent signal emanating from the hypothetical sphere will make the case that it is indeed the home of an advanced alien civilization.

This places a special burden on Dyson Sphere searches, which must, in fact be double searches. First, potential Dyson Spheres are identified by looking for stars emitting an excess of infrared radiation; next, these candidate spheres must be confirmed by conventional SETI searches, looking for intelligent transmissions. Few places in the world are well positioned to take on this double search. But one of these is SETI@home headquarters in Berkeley.

Dyson Spheres or Emerging Planets?

Charlie Conroy is an undergraduate physics student at U.C. Berkeley, who has developed a strong interest in all things SETI. Under the guidance of SETI@home chief scientist Dan Werthimer, he set out to search for telltale signs of Dyson Spheres using both publicly available resources, and those available only at Berkeley – one of the world centers of SETI research.

Charlie’s first step was to identify a pool of candidate stars that could potentially be the bearers of Dyson Spheres. Simply looking for stars with an excess of infrared radiation would not do, he realized, since in the years that have passed since Dyson made his proposal, such radiation had come to represent something quite different. Planetary systems, astronomers believe, are formed from giant swirling disks of gas and dust that sometimes surround young stars. Many such “protoplanetary disks” have been observed in the past twenty years, and all of them exhibit precisely the characteristic Dyson predicted for his sphere: they shine brightly in the infrared part of the spectrum. They are, in effect, “natural” Dyson Spheres formed without any intervention by intelligent beings.

In order to detect true (that is, artificial) Dyson Spheres, Charlie concluded that he must first exclude the “false positives” of protoplanetary disks. In this he was aided by the fact that as far as is known, such disks exist only around relatively young stars. This is because the star itself is formed from the disk at or near its center, and the gas and dust in the outer regions dissipate or accrete into planets. By the time a star reaches “middle age,” it may or may not have a planetary system, but its protoplanetary disk is in any case gone.

Charlie therefore decided to exclude young stars from his search, and include only those that would almost certainly not have disks of dust and gas swirling around them. Only stars one billion years of age or older, whose protoplanetary disk would most certainly have dissipated or condensed into planets, would be included in the search.

Finding a substantial list of stars whose ages are known, however, turned out to be a challenging task in itself. Fortunately, however, astronomers Geoff Marcy and Jason Wright, based, like Charlie, at U.C. Berkeley, had recently published a list of 1000 such stars, all within the galactic neighborhood of the Earth at a distance of up to 50 parsecs (163 light years). Marcy and Wright are extrasolar planet hunters – members of the most successful team in the world - and they compiled their list in order to identify stars that would be likely to be hosts to planetary systems. They were therefore interested in stars whose protoplanetary disk have had time to condense into planets, in other words – older stars. The list was therefore a perfect fit for Charlie’s Dyson Sphere search.

A Reddish Glow

Equipped with Marcy and Wright’s list, Charlie then turned to detecting any excess infrared radiation emanating form the stars. For this he consulted two of the most current star catalogues that carry this information: the IRAS Point Source Catalogue, based on the measurements of the InfraRed Astronomy Satellite; and 2MASS – the “Two Micron All Sky Survey” based at Caltech. Charlie found that 539 of his 1000 stars appear in the two catalogues, along with their levels of infrared radiation.

Based on calculations done in the 1990s by Japanese researchers J. Jugaku, S. Nishimura, and K. Noguchi, Charlie estimated that a Dyson Sphere would radiate at an excess temperature of around 300 degrees Kelvin. This in turn would translate to surplus radiation at the 12 micron wavelength, which is what one must look for in the candidate stars, Charlie concluded.

Determining an excess level of radiation, however, is not as simple as it sounds. First there is the problem that a star’s brightness diminishes with a star’s distance from the Earth. This makes it meaningless to directly compare levels of infrared radiation among different stars: a true Dyson Sphere, if it is very distant, will still emit weaker infrared radiation than a normal star if it is close by. Charlie solved this problem by measuring not the total 12 micron radiation, but the difference between the 12 micron level and the “K Band” level – “K Band” being a star's radiation at around the 2.2 micron wavelength. Since both the 12 micron and the K Band levels will be equally affected by the star’s distance, and because the K Band is particularly steady, the difference between the two provides a good indicator of the levels of 12 micron radiation.

Once Charlie had established the K-12 micron figures for each of the 539 stars on his list, he still had to decide on a crucial question: what K-12 radiation level should be considered “excess” radiation? To determine this, he relied on a simple statistical approach. All things being equal, the K-12 levels from the different stars would sort themselves in a normal, bell-shaped distribution. Most stars would be congregated around a mean figure that represents the most common level of K-12 radiation in older stars, while fewer stars would show radiation levels much higher, or lower, than this mean. The further one moves from the mean, the fewer stars will fit these levels of radiation.

When Charlie charted his results on a graph, he found that his 539 stars did indeed fit the normal distribution – almost. There on the right side of the graph, indicating a region with a strong 12 micron radiation there was a small but noticeable “bump.” Instead of the number of stars tending to zero in that region, as it should in a normal distribution, the “bump” represented a group of 33 stars whose infrared radiation level did not conform to the norm. Clearly, these 33 stars have an excess of infrared radiation in the 12 micron range.

Are these 33 stars all homes to advanced civilizations that built spherical shells around them? Probably not – the excess infrared radiation in most, if not all, of these stars can probably be explained by some unknown natural processes. Nevertheless, since the stars are over a billion years old and therefore do not possess protoplanetary spheres, it is not clear what these processes might be. For now, the 33 stars must be considered Dyson Sphere candidates.

Spheres and Signals

Now that Charlie had located a group of candidate stars, the next stage was to compare them to the results of traditional SETI searches. Are any of these stars a source of promising radio or light signals? It was at this point that the tremendous advantages of working at one of the world centers of SETI research came into play. For right there, based at the U.C. Berkeley campus, were not one, but three different SETI projects that could be used to test Charlie’s potential Dyson Spheres: SETI@home, SERENDIP IV, and Berkeley Optical SETI.

The first two are radio SETI projects, each with its own unique strengths. SETI@home collects radio data at Arecibo and sends it to be processed by millions of users around the world. It concentrates on a narrow radio band around the hydrogen line frequency, but analyzes it at a level of sensitivity unmatched by any other SETI project. SERENDIP IV uses the same Arecibo data, but analyzes a wider radio band at a lower sensitivity level. The Optical SETI project uses the 30 inch telescope at the Leuschner observatory to search for brief light pulses emanating from nearby stars.

Charlie made use of all three projects. He used the mountains of data processed by SETI@home users to look up 22 of the stars on his candidate to see if any significant radio signals have been detected coming from them. The other 11 were not, unfortunately, observable from Arecibo. He did the same with the data processed by SERENDIP IV, but when it came to optical SETI, sifting through data bases of processed signals was not enough. The candidate Dyson Spheres were actually observed directly by the Leuschner telescope to see if they were sending out light pulses. Fortunately for Charlie, his mentor Dan Werthimer is not only SETI@home’s chief scientist but also leader of the optical SETI project. With Dan’s help, setting up the observations was not difficult.

At the end of the day, no Dyson Sphere had been confirmed by any of the three searches. No unusual radio transmissions had been detected by SETI@home or SERENDIP at any of the 22 observable candidates stars; no pulsed light signals had been observed by the Leuschner telescope. If there are any Dyson Sphere among Charlie’s 33 candidates, they are either among the 11 unobservable stars, or their confirmation will have to await more sensitive detection methods. For the moment Charlie’s search has resulted in 33 stars whose infrared radiation patterns are a mystery. Are any of them the home of an advanced technological civilization, which wrapped itself in a solid shell in order to exploit the energy of its star? Right now we simply do not know.